Dispersing agents for suspension polymerizations are known, e.g. polyvinyl alcohols and partially saponified polyvinyl acetates or cellulose ethers such as hydroxyethyl cellulose or sodium carboxymethyl cellulose. Especially suitable dispersing agents are homopolymers of acrylic acid or of methacrylic acid and particularly suitable are copolymers, at least 50% by weight of which is derived from these acids. These polymers can be used in the form of free acids or as alkaline-metal salts or ammonium salts. They have the advantage that they can generally be readily removed by washing the polymer beads which are formed in the process.
However, this advantage is associated with a significant disadvantage, i.e., that undesirably large amounts of very fine emulsion polymer are produced when using these dispersing agents in addition to the desired suspension polymer (see Makromoleculare Chemie 78, p. 25, (1954)). This emulsion polymer remains during the workup of a polymerization batch, for the most part in the waste water, and consequently results in a corresponding yield loss. Since the emulsion polymer deviates generally from the desired suspension polymer in its molecular weight and, in the case of co-polymers, also in the monomer composition, recovery from the waste water for the purpose of improving the yield is generally not useful.
Japanese published, unexamined application 2302/83 suggests preventing the formation of emulsion polymer by the addition of water-soluble metal salts of polyvalent carboxylic acids in an amount of 100 to 20,000 ppm relative to the aqueous phase. However, an experimental test showed no, or at the most an unsatisfactory, effect in many instances.